1. Field of the Invention
The present invention relates to an electric power converter suited for converting heavy electric power, and especially to a large capacity power converter with improved snubber circuits.
2. Description of the Related Art
A voltage source inverter, which comprises a three-phase bridge circuit employing gate turn-off thyristors (GTOs) and snubber circuits provided across a positive arm and a negative arm of every phase, is discussed in the article "PROGRESS IN VOLTAGE SOURCE INVERTERS (VSIs) AND CURRENT SOURCE INVERTERS (CSIs) WITH MODERN SEMICONDUCTOR DEVICES" by T. Salzmann et al, IEEE, IAS, Conference Record of Annual Meeting (1987), pages 577 to 583. A snubber circuit as disclosed in this article is known as the Marquard low-loss snubber circuit. However, this article only discusses the case, in which the Marquard low-loss snubber circuit is applied to a three-phase bridge circuit, each arm of which is composed of a signal GTO.
As is well know, in a large capacity power converter, each of arms thereof has some GTOs connected in series. In such a case, when one of the arms is commutated to another arm, all of GTOs in the one arm are not always turned off simultaneously because of the difference in turn-off time characteristics thereof. Namely, there can often occurs the case where one of GTOs of an arm still remains conductive, whereas another GTO of the arm has been already turned off.
Under such a circumstance, equivalent capacitance of the another GTO is charged by a current which still flows through the one GTO due to inductance of a wiring of the arm. Since such equivalent capacitance is usually very small, the voltage appearing across the another GTO rapidly increases and may result in an overvoltage, before the one GTO is turned off.
Therefore, the prior art snubber circuit as mentioned above is not suited or insufficient for use in such a large capacity power converter that each arm of the converter is composed of plural GTOs connected in series.